A dust-proof covering element is provided to close a projection aperture located in the dashboard of a vehicle and through which the information-carrying source light beam projected by the projecting module of a head-up display device passes on its way to a partially reflective return element. The covering element comprises a transparent carrier and an optical coating placed on one of the faces of the carrier; wherein, the optical coating comprises a metal layer with plasmonic properties, said metal layer being endowed with an array of subwavelength nanoperforations, and being configured to allow the extraordinary transmission of one or more restricted bands of wavelengths of the visible spectrum that are centered on the one or more wavelengths of the light beam projected by the projecting module.